There was an article in Men's Health about the Myth of the Ectomorph. http://www.menshealth.com/fitness/skinny-man-muscle
The main knowledge nugget from this article is that:"Underweight people tend to overestimate their daily calorie intake," Aragon says. "Then they incorrectly attribute their low weight to a high metabolism."
How much do you want to weigh? Do you know how many calories that requires?
Go to the site below, where you can see the number of calories needed to adjust your weight (You can even put in a date for a goal and it will tell you how many calories to eat to be a certain weight by that date)http://caloriecount.about.com/cc/calories-goal.php
How many calories do you really get per day? You can calculate for a week and see.
I have a food scale to weigh out food according to the label's serving sizes (actually a $10 postal weight scale from Walmart), and google items that don't have nutrition information labels (like a peach). I used to think I consumed way more calories than I actually did eat. Now I have an excel spreadsheet that I plug in my calories for the day, and see if I hit my mark. After awhile, it is habit to eat that amount. (Tip: Try not to gain more than 2lbs a week, because that may just contribute to excess body fat and can be bad for your body.)
I hope this helps!